Add full-stack BTC portfolio web app

Multi-user FastAPI + React app with JWT auth, SQLite storage, and
CoinGecko price integration. Dockerized with docker-compose.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

btc-portfolio/backend/app/main.py

@@ -0,0 +1,26 @@
+from fastapi import FastAPI
+from fastapi.middleware.cors import CORSMiddleware
+
+from .database import engine, Base
+from .routes import users, purchases, stats
+
+Base.metadata.create_all(bind=engine)
+
+app = FastAPI(title="BTC Portfolio API")
+
+app.add_middleware(
+    CORSMiddleware,
+    allow_origins=["http://localhost:3000"],
+    allow_credentials=True,
+    allow_methods=["*"],
+    allow_headers=["*"],
+)
+
+app.include_router(users.router)
+app.include_router(purchases.router)
+app.include_router(stats.router)
+
+
+@app.get("/")
+def root():
+    return {"message": "BTC Portfolio API"}